A) Field of the Invention
This invention relates to a treatment method for the internal surface of a moulded polyethylene plastics material container, and also to such containers whenever treated by the method of this invention.
B) Description of the Prior Art
Moulded plastics material containers are very widely used in industry for the storage and transport of very many different products including liquids, powders, granules and other flowable products. As compared to steel containers, moulded plastics material containers have several advantages, including corrosion resistance, resilience restoring the original shape if distorted, resistance to bursting, electrical and thermal insulation, and the ability to be self-coloured. However, the industrial acceptance for the storage and transport of solvents, solvent-containing products and various chemicals has been limited since the plastics materials from which containers are made are susceptible to attack by various chemicals encountered in industry.
Plastics material containers are being used more widely as various techniques are developed for increasing the resistance to chemical attack of the materials from which the containers are made, or by developing barrier layers for coating on the container walls to isolate the material of the container from contained substances. For example, both small domestic petrol cans and motor vehicle petrol tanks are now made from plastics materials, and demonstrate sufficient insolubility and resistance to puncturing for such containers to present no greater risk than would a steel container for petrol.
A particular problem arises in the case of containers moulded from polyethylene. Many industrial solvents can attack polyethylenes, and attempts to coat the surfaces of a polyethylene container with solvent-resistant materials have largely produced unacceptable results. The surface of moulded polyethylene is not at all receptive to conventional coating compositions used to form barrier layers and even should a suitable composition be deposited on the surface, the adherence of that composition tends to be very poor, leading to localised breakdown. This is especially so if the container walls are flexed either deliberately or accidentally, by more than some relatively small amount. The term polyethylene as used throughout the specification is intended to encompass pure polyethylene as well as mixtures of polymers which include polyethylene or polyethylene together with other substances such as fillers or reinforcing agents.
A further problem associated with the industrial use of polyethylene containers is that it is very easy for a static charge to build up on a container when the container is being transported, filled or emptied with a product, consequent upon product friction and/or handling (tribocharging). This can occur with either powders or liquids.